This was reported in SERVER-2051, but appears to be a bug of its own (although the problem might be masked if the change in that ticket was made):

> db.left.insert(

{field1: "whatever", "": null}

)
> db.left.update(

{"field1": "whatever"}

, {$set: {"field2": "test"}}, false, true)
LEFT_SUBFIELD only supports Object: not: 10

On a quick look at the source, it looks like the result of compareDottedFieldNames is incorrect when r is the null string.

Probably related:

> db.left.insert(

{field1: "whatever"}

)
> db.left.update(

{"field1": "whatever"}

, {$set: {"": "test"}}, false, true)
ModSet::createNewFromMods - RIGHT_SUBFIELD should be impossible